Such switches are used for electric tools with an electric motor, for example for handheld electric tools, such as electric drills, hammer drills, electric screwdrivers or the like.
Such an electric switch, which is known from DE 41 14 854 A1, has a switch housing with first electrical terminals for the electrical connection to a voltage supply and second electrical terminals for the electrical connection to the electric motor. The switch has an actuating mechanism which can be adjusted between an initial position and a final position manually by the user, wherein the electric motor is switched off when the actuating mechanism is located in the initial position and is switched on when the actuating mechanism is not located in the initial position, i.e. when the actuating mechanism is actuated by the user, for operation thereof. If appropriate, the switch can have a control device for operating, and specifically for the open-loop and/or closed-loop control of, the electric motor, such as the speed, torque or the like thereof, depending on the displacement path of the actuating mechanism. Furthermore, the switch is provided with a braking circuit for braking the electric motor, said braking circuit operating when the actuating mechanism is reset to the initial position. Finally, a reversal circuit for switching over the direction of rotation of the electric motor is also arranged in the switch.
In the case of the known switch, the mechanical braking circuit is realized, by means of an electromechanical switching contact, in the form of a braking contact which is arranged in the switch housing electrically upstream of the mechanical changeover switch for the direction of rotation and the electrically downstream motor terminals. The same arrangement is also possible in principle when using an electronic brake instead of the braking contact. In a normal case, the changeover switch for the direction of rotation of the electric motor is switched while the electric motor is switched off, i.e. in the deenergized state. In the event of very rapid actuation of the reversal circuit by the user, specifically even before the braking operation has come to an end, it may also arise, however, that the switchover contact of the reversal circuit interrupts the induced braking current as it is switched over. In this case, the electric motor continued to run, with the brake being arranged downstream of the changeover switch when seen from the motor. Then, the changeover switch switches on the electric motor again unintentionally since the blocking diode operates in the forward direction as a result of the changeover which has taken place, for example.
In this specific case, the changeover switch is therefore not a no-load switch as it is switched over. As a result, the changeover switch in the reversal circuit, as well as the other mechanical contacts switching on load in electrical switches, is subjected to mechanical and electrical wear as a result of abrasion of and/or a change in the contact material over the life of said changeover switch. This wear takes place primarily as a result of the arc ripping away during switching-off, but also during switching-on. The wear of the contact material over the life results in an increase in the contact resistance and therefore in possibly impermissible heating of the contact points in the reversal circuit. Given correspondingly high currents, this can lead to further wear and/or welding of the contacts. At the latest when the contacts have been completely worn, this results in failure of the switch. In addition, for reasons of costs in electric tool switches most of the changeover switches for the direction of rotation are configured in the form of sliding switches which are normally designed for off-load switching, with the result that primarily the abovementioned switchover operations are very damaging during the braking operation for the changeover switch.